Flow control and reduced-order modelling of transition in shear flows
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
In this thesis direct numerical simulation is used to investigate the possibility to delay the transition from a laminar to a turbulent flow in boundary layer flows. Furthermore, modal analysis techniques are used to identify the coherent structures in wind turbine wake.
Among different transition scenarios, the classical transition scenario is considered where the laminar-turbulent transition occursdue to Tollmien-Schlichting (TS) waves. These waves are convectively unstable and when triggered inside the boundary layer, they grow exponentially inamplitude as they are advected downstream of the domain. The aim is to suppressthese waves using flow control strategies based on a row of spatially localised sensors and actuators distributed near the wall inside the boundary layer. To avoid the high dimensionality arising from discretisation of the Navier–Stokes equations, a reduced order model (ROM) based on the Eigensystem Realisation Algorithm(ERA) is obtained and based on that a linear controller is designed. To manip-ulate the flow, a plasma actuator is modelled and implementedas an externalforcing. To account for the restrictions of the plasma actuators, several strategies are proposed and tested within the LQG framework. We studied also the design of a faster controller based on decentralised approach and compared the performance to a more expensive centralised controller. The outcomes revea lsuccessful performance in mitigating the energy of the disturbances inside the boundary layer and suppressing the TS waves.
To extract coherent features of the wind turbine wakes, modal decomposition techniques are employed. In this method a large dynamical system is reduced to a fewer number of degrees of freedom. Two decomposition techniques are employed, namely proper orthogonal decomposition and dynamic mode decomposition. In the former, the flow is decomposed into a set of orthogonal structures which are ranked according to their energy contents in a hierarchical manner. In the latter, the eigenvalues and eigenvectors of the underlying approximate linear operator of the system is evaluated. In particular each mode is associated with a specific frequency and growth rate. The results revealed the coherent structures which are dynamically significant for the onset of instability in the wind turbine wake
Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , vii, 57 p.
TRITA-MEK, ISSN 0348-467X ; 2014:14
Flow control, plasma actuator, wing, leading edge, flat plate, wind turbine, optimal controller, model reduction, proper orthogonal decomposition, dynamic mode decomposition.
Fluid Mechanics and Acoustics
IdentifiersURN: urn:nbn:se:kth:diva-145631ISBN: 978-91-7595-170-6OAI: oai:DiVA.org:kth-145631DiVA: diva2:719319
2014-06-10, Sal F3, Lindstedtsvägen 26. KTH, Stockholm, 10:15 (English)
Theofilis, Vasssilis, Professor
Henningson, Dan, Professor
QC 201405262014-05-262014-05-232014-05-26Bibliographically approved
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